Optical spectrum analyzers typically comprise a tunable filter that bandpass filters an input signal. A detector is used to measure the filtered signal and thereby determine the optical power within the filter""s current pass band.
Analyzers have relevance to many applications requiring optical spectrum analysis. Such applications span LIDAR to remote chemical analysis, for example. Presently, however, one of the most active applications is in the area of data transmission systems utilizing wavelength division multiplexing (WDM).
WDM systems typically comprise multiple separately modulated laser diodes at the transmitter. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. At the receiving end, the channels are usually separated from each other using thin film filter systems, for example, to thereby enable detection by separate photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data for WDM systems is typically less than comparative non-multiplexed systems. This is because any amplification system required along the link can be shared by all of the separate channels transmitted on the link. With non-multiplexed systems, each channel/fiber would require its own amplification system.
The economics pulling for WDM in the context of long-haul optical links is only one factor suggesting the long-term applicability of the technology. Another application concerns the dynamic routing of individual wavelength slots or channels. This is sometimes referred to as metro WDM.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, additional optical components are required to combine the channels into, and separate out the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels lose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
In order to ensure that proper guard bands are maintained between adjacent channels and to also ensure that the carrier frequencies or wavelengths of the channels are correct both relative to other channels and relative to their wavelength assignments, optical monitoring systems are required in most WDM transmission systems. They are also useful in WDM channel routing systems, such as add/drop multiplexers and switches to ensure that the specific optical channels are being properly controlled. Further, information concerning the relative and absolute powers in the optical channels is important as feedback to variable attenuators, for example, and to combat gain tilt.
One problem associated with these spectrum analyzers is their expense, which slows their advance toward the network edge and adds incrementally to the deployment costs for WDM systems, for example. However, as the operation and stability of WDM systems are refined and as the speed of spectrum analyzers is increased, it becomes less necessary for a spectrum analyzer to be dedicated to monitoring each optical link.
The present invention concerns an optical spectrum analyzer that includes an integrated beam switch array. As a result, a single spectrum analyzer can be amortized across multiple optical links with pigtails transmitting the optical signals from separate optical links to the analyzer. The switch array provides one of the optical signals as an input signal to the optical spectrum analyzer. Further, the invention also concerns an analyzer with this functionality that is further capable of being integrated into a small package to be used as a subsystem, or possibly even as a stand-alone system, in a WDM system, or other application requiring optical spectral monitoring.
In general, according to one aspect, the invention features an optical spectrum monitoring system. The system comprises a bench and fiber pigtails terminating at the bench. A tunable filter is further connected to the bench and functions to filter an input optical signal. An array of beam switches, connected to the bench, selectively provides the optical signals from the fiber pigtails as the input optical signal to the tunable filter.
In general, according to another aspect, the invention features a method. Specifically, the method comprises providing optical signals to an optical bench and selectively providing one of the optical signals to a tunable filter. A filtered signal from the tunable filter is then detected.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.